Web edge sensing and guiding apparatus

ABSTRACT

A web edge sensing and guiding system in which a travelling web exits from a tiltable web guide apparatus in a plane transverse to the plane of motion of the web guide apparatus. The lateral position of the web edge is sensed at the exit side of the web guide apparatus and means are provided for moving the web edge sensor along a path to substantially follow movement of the exiting web in a direction perpendicular to its plane while maintaining the web edge sensor at a substantially uniform spacing laterally of the longitudinal path of travel of the web to thereby accommodate fluctuation of the exiting web in a direction perpendicular to its plane caused by tilting of the web guide apparatus in its plane of motion.

United States Patent Ott, Jr.

[ 1 Aug. 8, 1972 1541 WEB EDGE sENs1NG AND GUIDING APPARATUS [72] Inventor: Robert W. Ott, Jr., Rockford, Ill.

[73] Assigneez Rockford Servo Corporation,

Rockford, Ill.

[22] Filed: March 15, 1971 [21] Appl. No.: 124,205

52 U.s.'c1. .Q ..226/21 511 111801. ..B65h 25/26 [58] FieldofSearch ..226 21,22, 23

[56] References Cited I I UNITED STATES PATENTS 3,615,048 10/1971 9 Martin ..226/21x 3,411,683 11/1968 Bartlesetal ..226/21 3,581,963 6/1971 Rule ..226/21 Primary Examiner-Allen N. Knowles Attorney-McCanna, Morsbach, Pillote & Muir [57] ABSTRACT A web edge sensing and guiding system in which a travelling web exits from a tiltable web guide apparatus in a plane transverse to the plane of motion of the web guide apparatus. The lateral position of the web edge is sensed at the exit side of the web guide apparatus and means are provided for moving the web edge sensor along a path to substantially follow movement of the exiting web in a direction perpendicular to its plane while maintaining the web edge sensor at a substantially uniform spacing laterally of the longitudinal path of travel of the web to thereby accommodate fluctuation of the exiting web in a direction perpendicular to its plane caused by tilting of the web guide apparatus in its plane of motion.

9Claims, 6DrawingFigures BACKGROUND It is common practice in guiding travelling webs to utilize a web guide apparatus mounted for tilting movement relative to the path of web travel and to sense the lateral position of the web and automatically tilt the web guide apparatus in a direction to correct the deviation of the web from the desired path of web travel. It is generally desirable to sense the position of the travelling web as it leaves the web guide apparatus so that the sensor responds to the corrected path of travel web edge at a location where the web is subjected to substantial movement in a direction normal to its plane and the apparatus maintains the web edge sensor spaced at generally uniform distance from the plane of the web at the sensing location.

An important object of this invention is to provide a web edge sensing and guiding apparatus in which the I lateral position of the web edge is sensed at a location produced by the web guide apparatus. However, when the web leaves the web guide apparatus at a substantial angle to the plane of motion of the web guide apparatus, the exiting web is not only shifted laterally in the plane of the exiting web by the web guide apparatus to correct lateral deviation of the web from the desired path of web travel, but the web is also subjected to a shifting in a direction normal to the plane of the exiting web, which last mentioned shifting of the web is herein referred to as pass-line fluctuation of the web. Substantial movement of the web in a direction normal to its plane markedly complicates the problem of sensing the position of the web. The sensor must not only be such as to physically accommodate the maximum passline fluctuation of the web, but must also produce a signal which is correlative with the lateral movement of the web in its plane, but substantially independent of the shifting movement of the web in a direction normal to its plane. Some sensors, such as gap-type web edge sensors, can accommodate only limited shifting of the web in a direction normal to its plane, and some others, such as vacuum type sensors, can accommodate almost no shifting of the web normal to its plane. Y t

In order to minimize pass-line fluctuation at the web edge sensor, it has heretofore been the common practice to locate the web edge sensor a substantial distance downstream of the web guide apparatus. However, this merely reduces but does not eliminate the pass-line fluctuation of the web at the sensor. Moreover, when the sensor is located a substantial distance downstream from the web guide apparatus, the web guide apparatus must over-correct or over-shoot before the web edge sensor is satisfied, and this tends to cause hunting of the web guide apparatus.

SUMMARY OF THE INVENTION to substantially follow the component of movement of I the exiting web in a direction perpendicular to the exiting plane of the web while maintaining the sensor at a substantially uniform spacing laterally of the desired path of web travel to thereby accommodate pass-line fluctuation of the exiting web caused by tilting of the web guide apparatus in its plane of motion. This arrangement allows sensing of the lateral position of the after the web exits from the web guide apparatus and which maintains the web edge sensor spaced a generally uniform distance from the plane of the exiting web.

A more particular object of this invention is to provide a web edge sensing and guiding apparatus having a web guide apparatus tiltable in aplane'of motion and a web edge sensor for sensing the lateral position of the web as it exits from the web guide apparatus and means responsive to tilting movement of the web guide apparatus for shifting the web edge sensor in a direction generally perpendicular to the plane of the exiting web to follow pass-line fluctuation of the exiting web while maintaining the sensor at a predetermined lateral position with respect to the path of travel of the web.

Another object of this invention is to provide a web edge sensing and guiding apparatus in accordance with the foregoing object and which is adjustable for use with webs of different widths. v

These, together with other objects and advantages of this invention will be more readily understood by reference to the following detailed description when taken in connection with the accompanying drawings therein:

FIG. 1 is a fragmentary perspective view of a web handling apparatus embodying a web guide apparatus constructed in accordance with the present invention;

FIG. 2 is a plan view of the web guide apparatus shown in FIG. 1;

FIG. 3 is a fragmentary plan view illustrating the web guide apparatus in different moved positions;

FIG. 4 is an end view of the web guide apparatus shown in FIGS. 1-3;

FIG. 5 is a fragmentary view illustrating a modified embodiment of the present invention;

FIG. 6 is a fragmentary side view taken on the plane 6-6 of FIG. 5.

The present invention relates to a web edge sensing and guiding apparatus wherein a web W is laterally guided along a predetermined longitudinal path of web travel by a web guide apparatus that is tiltable relative to the desired path of web travel in a manner to substantially maintain the web in the desired path. In the preferred embodiments of the web guide apparatus shown in the accompanying drawings, the web guide apparatus is of the type which engages the travelling web inter-mediate its ends to laterally guide the web in response to tilting of the web guide apparatus. It is contemplated, however, that the web guide apparatus could be in the form of a web unwind roll stand which supports a web roll for tilting movement in a plane of motion.

Referring now more specifically to the accompanying drawings, the web guide apparatus, designated generally by the numeral 12, includes a movable guide frame 23 that is mounted 'on a stationary support structure 22 for tilting movement relative to the desired longitudinal path of web travel. The guide apparatus also includes a web edge sensor 16 located to sense the lateral position of the web at a location after the web exits from the web guide apparatus 12, and a control apparatus 17 responsive to the signal produced by the sensor 16 for operating an actuator 18 to tilt the web guide apparatus in a direction to correct deviation of the web from the desired path of web travel. The sensor 16 and control apparatus 17 can be constructed and arranged for either edge guiding the web or center-line guiding of the web, as desired. Thus, if the web is laterally offset in one direction, such as to the right of the desired path of web travel, the control apparatus operates to tilt the web guide apparatus in the other direction, that is to the left to correct the deviation of the web from the desired path of travel. In the intermediate type web guide apparatus illustrated, the web guide apparatus is located between spaced element such as rollers and 11 of a web handling system to laterally guide and position the web W as it passes from roller 10 to roller 11. As shown in FIG. 1, rollers 10 and 11 are supported on a part 220 of the stationary support structure 22 and the movable guide frome 23 is tiltable relative to the rollers 10 and 1 1.

Various different arrangements can be utilized to support the web guide apparatus for tilting movement. In the embodiment shown, the web guide apparatus has an exiting web guide roller mounted thereon and web guide apparatus is supported so that, during tilting movement thereof, the exiting web guide roller 15 is disposed generally tangent to an arcuate path having an effective are center designated P and located in advance of the main web guide roller 15. The particular mechanism herein shown for supporting the web guide apparatus for tilting movement is of the type generally disclosed in the US. Pat. to Robert W. Ott, Jr. No. 3,390,823 and which includes a plurality of arcuate guide bars 21 mounted on the stationary frame structure 22 and constructed and arranged so as to have a common arc center at P. Followers 24 are slidably mounted on each of the arcuate guide bars 21 and support the movable frame structure 23 for movement about the arc center P. The exiting web guide roller 15 is rotatably supported as by a shaft 15a on the movable frame for tilting movement therewith in the plane of motion from its neutral position perpendicular to the path of web travel. In the embodiment shown, the effective pivot point P of the web guide apparatus is spaced a substantial distance in advance of the exiting web guide roller. The invention, however, is not limited to such web guide apparatus wherein the elTective pivot point is spaced a substantial distance from the exiting web guide roller and may, for example, be applied to web guide apparatus in which the exiting roller is tilted about an axis closely adjacent or even intersecting the axis of the exiting web guide roller. Moreover, while arcuate guide and followers 21, 24 are herein shown provided for supporting the web guide apparatus for tilting movement in a plane of motion, it is to be understood that various different arrangements can be utilized to support the web guide apparatus 12 for such tilting movement in a plane of motion.

In order to have the web edge sensor automatically respond to corrections in the path of web travel introduced by the web guide apparatus, it is generally desirable to sense the lateral position of the web as it exits from the web guide apparatus. However, when the web exits from a tiltable web guide apparatus in a direction transverse to the plane of motion of the web guide apparatus, the web is not only shifted laterally in the plane of the web by the tilting web guide apparatus, but it is also shifted in a direction normal to the plane of the web. This last mentioned shifting motion, in a direction normal to the plane of the web as it exits from the web guide apparatus is herein referred to as passline fluctuation of the web. The pass-line fluctuation varies as a function of the angle through which the web guide apparatus is tilted and the width of the web. In many installations, particularly in those involving relatively wide webs or web guide apparatus which tilt through substantial angles, the pass-line fluctuation becomes quite large. Many web edge sensors can only accommodate a very limited pass-line fluctuation and, in many web guide apparatus wherein the sensors were arranged to sense the lateral position of the web after it exits from the movable web guide apparatus, the web edge sensors were located downstream a substantial distance from the movable web guide apparatus at a point where the pass-line fluctuation was minimized. However, as previously pointed out, locating the web edge sensor downstream a substantial distance form the web guide apparatus increases the tendency of the web guide apparatus to hunt, particularly in slow speed web handling systems.

In the present invention, the web edge sensor 16 is located to sense the position of the web edge after the web leaves the main web guide apparatus and preferably at a point relatively closely adjacent to the tiltable web guide apparatus so that the sensor rapidly responds to changes in the lateral position of the web edge introduced by the web guide apparatus. In accordance with the present invention, provision is made for moving the web edge sensor in a direction to sub stantially follow movement of the exiting web at the sensing location in a direction perpendicular to the plane of the exiting web while maintaining the sensor at a substantially uniform spacing laterally of the desired longitudinal path of web travel to thereby accommodate pass-line fluctuation of the exiting web caused by tilting of the web guide apparatus in its plane of motion. As best shown in FIGS. 1 and 2, the web edge sensor 16 is mounted on the stationary support structure 22 for movement relative to the stationary support structure along a substantially straight line path, by means of a guide rod 31 and followers 32. As shown, the guide rod 31 is attached to the stationary support structure and has a non-circular cross section so as to nonrotatably support the followers 32 for straight line movement therealong. The axis of the guide rod extends generally perpendicular to the plane of the exiting web designated WX, when the main web guide apparatus 12 is in its neutral or mid-position, and the axis of the guide rod generally parallels the longitudinal path of the travel of the web to be maintained by the web guide apparatus. As will be understood, the web W as it passes through a web handling system, generally passes around a number of rollers and other instrumentalities which operate on the web and frequently travels in many different directions both vertically and horizontally and combinations of the two. However,

the over-all longitudinal path of the travel of the web as it passes through the apparatus must be maintained in a predetermined path as the web passes certain instrumentalities and the web guide apparatus is arranged to correct deviations of the web from this predetermined path of web travel. The exiting web guide roller 15, when in its mid-position, is disposed generally perpendicular to the desired path of web travel and, when the plane of the exiting web WX is substantially perpendicular to the plane of motion of the web guide apparatus 12, the axis of the guide rod 31 generally parallels a line designated L through the effective pivot P and perpendicular to the exiting web guide roller.

As best shown in FIGS. 1 and 2, the web edge sensor 16 is mounted on the followers 32 for straight line movement therewith along a path paralleling the longitudinal path of travel of the web through the web handling system. The web edge sensor can be of various types and, in the embodiment shown, comprises a gaptype pneumatic sensor having arm portions 160 and 16b at relatively opposite sides of the web and arranged to transmit a jet of air across the web edge, which air jet is variably interrupted by the web edge to produce a pneumatic signal at one of the arms that is transmitted to the control apparatus 17 for operating the actuator 18 to shift the main web guide roller 15 in a direction to correct deviation of the web from the desired path of web travel. The gap-type sensing and control apparatus may, for example, be of the type shown in the US. Pat. to Robert W. Ott, Jr., No. 3,407,706 issued Oct. 29, 1968. It is to be understood, however, that other types of web edge sensors such as photo-electric sensors, vacuum type sensors and feeler type sensors can be used, with correlated control apparatus if desired.

Provision is made for automatically shifting the web edge sensor 16 in response to tilting movement of the main web guide roller 15 so as to cause the sensor to automatically follow the pass-line fluctuation of the exiting web WX. In the web guide apparatus illustrated, the guide apparatus also includes an inlet web guide roller 41 mounted as by shaft 41a on the movable guide frame 23, which inlet guide roller parallels the exiting guide roller 15 and is located closely adjacent the effective arc center P of the tiltable web guide apparatus. Further, the stationary guide roller is preferably ar ranged with respect to the guide apparatus so that the plane of the entering web WE is disposed substantially coincident with the effective are center P. The outlet stationary roller 11 is located with respect to the exiting web guide roller 15 so that the plane of the exiting web WX extends transverse to the plane of motion of the web guide apparatus and, in the form shown, substantially perpendicular to the plane of motion of the web guide apparatus. The apparatus for shifting the sensor 16 along the sensor shift path of the rod 31 is advantageously operatively connected to the movable guide frame to effect shifting of the sensor in a direction and through a distance or amplitude correlative with the pass-line fluctuation of the web. In general, the pass-line fluctuation of the exiting web WX varies as a function of the tangent of the angle through which the exiting web guide roller 15 is tilted from its neutral or mid-position. A linkage is provided for operatively connecting the web edge sensor to the movable frame structure and is connected to the movable guide frame at a point on the guide frame that moves through a distance correlative with the pass-line fluctuation of the exiting web produced by tilting of the guide frame. This linkage comprises an elongated bar 45 attached at one end by a pivot 46 to the followers 32 which support the sensor and attached at its other end by a pivot 47 to a bracket 48 on the movable frame structure, at a point laterally offset from the effective are center about which the main web guide roller tilts. As will be understood from a consideration of FIG. 2, the motion of the bracket 48 on the movable guide frame varies as a function of the tangent of the angle through which the main web guide roller is tilted and, moreover, the arcuate path through which the bracket 48 moves, generally parallels the sensor shift path of the guide rod 31 so that the sensor is shifted in a direction and through a distance correlative with the tangent of the angle through which the exiting web guide roller 15 is tilted. Preferably the pivots 46 and 47 are located relatively close to the side edge of the web to be guided so that the amplitude of movement of the web edge sensor is substantially equal to the pass-line fluctuation of the web. However, for relatively narrower webs or webs wherein the edge is spaced a substantial distance inwardly from the pivot points 46 and 47, a modified form of sensor support shown in FIGS. 6 can be advantageously provided.

The web guide apparatus and sensor shift mechanism of the embodiment of FIGS. 5 and 6 is similar to that described in connection with the embodiment of FIGS. 1-4 and like numerals are used to designate the same parts. In this embodiment, however, the web edge sensor is not mounted directly on the followers 32 but is instead mounted for adjustment laterally relative to the longitudinal path of web travel to adapt the web guide apparatus for handling webs of different widths. The amplitude of the pass-line-fluctuation varies with the web width and the sensor mounting arrangement is also arranged to reduce the amplitude of movement of the web edge sensor as the sensor is adjusted inwardly for narrower webs. As shown in FIGS. 5 and 6, an elongated arm 51 is pivotly mounted at one end as by a pivot pin 52 on the followers 32 and the arm extends crosswise of the path of web travel to a point preferably adjacent the lengthwise center of the exiting web guide roller 15. The inner end of the arm 51 is swingably supported at a location preferably approximately medially between the ends of the roller 15. This means includes a pivot pin 54 shown in FIG. 6 attached to a crossmember 22b of the stationary support structure 22. Since the pivot pin 52 for the outer end of the arm moves in a straight line path, the spacing between the pivot pins 52 and 54 changes during angular movement of the arm 51 and provision is made for accommodating this change in effective spacing adjacent the end of the arm. As shown in FIG. 6, the connector member 53 on the inner end of the arm is provided with an elongated opening 53a to form a slideable pivot for the inner end of the arm, it being apparent that a plain pivot together with a telescoping joint could be provided in the inner portion of the arm 51, if desired.

The web edge sensor 16' is mounted by brackets 56, 57 on the arm 51 for adjustment therealong crosswise of the path of web travel. A means such as a locking screw 58 is provided to selectively lock the sensor in its adjusted position. As will be seen, this arrangement allows the sensor to be adjusted laterally of the path of web travel to accommodate webs of different widths and, moreover, progressively decreases the amplitude movement of the web edge sensor along the guide rod 31, as the sensor 16 is adjusted inwardly for narrower webs.

From the foregoing it is thought that the construction and operation of the web guide apparatus will be readily understood. The web edge sensor is located to sense the lateral position of the web edge as it exits from the tiltable web guide roller and the sensor is moved automatically in response to tilting movement of the web guide roller in a direction to substantially follow passline fluctuation of the web while maintaining the sensor at a preselected lateral position with respect to the path of web travel. Thus, as shown in FIG. 3, the web guide apparatus 12 and exiting web guide roller 15 are shown in solid lines in the neutral position of the web guide apparatus and the web guide apparatus is tiltable relative to the path of web travel to tilt the exiting web guide roller to the left, as shown in phantom at 15L, and to the right, as shown in phantom at 15R, to laterally shift the edge of the web W respectively to the left and to the right for correcting deviation of the web edge from the desired path of web travel. During lateral shifting of the web, the exiting web is also shifted in a direction perpendicular to the plane of the exiting web. However, the pivot 47 on the movable guide frame also shifts during tilting of the web guide apparatus to moved positions shown in phantom in FIG. 3 at 47L and 47R respectively to thereby shift the web edge sensor along the sensor shift path determined by guide rod 31 to maintain a substantially uniform spacing between the sensor and the plane of the exiting web.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a web handling system for laterally guiding a web along a predetermined longitudinal path of travel in which the web engages a web guide apparatus mounted on a stationary support structure for tilting movement relative thereto in a plane of motion from a mid-position and exits from the web guide apparatus in an exiting plane transverse to the plane of motion of the web. guide apparatus and wherein a web edge sensor is positioned at a sensing location to sense the lateral position of an edge of the web in said exiting plane and to operate a power means for tilting the web guide apparatus in said plane of motion in a direction to correct deviation of the web edge from a preselected lateral position, the improvement comprising means defining a rectilinear guideway on said stationary support structure paralleling said predetermined longitudinal path of web travel and extending transverse to the exiting plane of the web, means including a follower slidable along said guideway for supporting said web edge sensor, and means responsive to movement of said web guide apparatus in said plane of motion for moving said follower along said guideway in a direction and distance to cause said web edge sensor to substantially follow the component of movement of the exiting web at the sensing location that is substantially perpendicular to the exiting plane of the web while maintaining the sensor at a uniform spacing laterally of said predetermined longitudinal path of travel to thereby accommodate pass-line fluctuation of the exiting web caused by tilting of said web guide apparatus in said plane of motion.

2. In a web handling system including a movable web guide apparatus mounted on a stationary support structure for tilting movement relative thereto in a plane of motion from a mid-position, and wherein the web exits from the web guide apparatus in an exiting plane transverse to said plane of motion, a web edge sensor at a sensing location after the web exits from said movable web guide apparatus to sense movement of the web edge laterally in said exiting planefrom a preselected lateral position, and power means operated by the web edge sensor for tilting the movable web guide apparatus in a direction to correct lateral deviation of the web edge from said preselected lateral position, the improvement comprising means defining a rectilinear guideway on said stationary support structure paralleling said predetermined longitudinal path of web travel and generally perpendicular to the exiting plane of the web, means including a follower slidable along said guideway mounting the web edge sensor for substantially straight line movement along a sensor shift path which is generally perpendicular to said exiting plane when the web guide apparatus is in its mid-position, and means for moving said sensor along said sensor shift path as said movable web guide apparatus is tilted in said plane of motion to substantially follow move ment of the edge of the exiting web in a direction perpendicular to said exiting plane to thereby accommodate pass-line fluctuation of the web at the sensing location.

3. A web handling system according to claim 2 wherein the movable web guide apparatus has a movable support frame mounted on said stationary support structure for tilting movement in said plane of motion, and said means for moving the web edge sensor includes linkage means operatively connected to said follower and to said movable support frame.

4. A web handling system according to claim 2 wherein the movable web guide apparatus includes an exiting web guide roller mounted by a movable support frame for tilting movement in said plane of motion about an effective arc center perpendicular to said plane of motion and spaced from said exiting web guide roller, said means for moving said web edge sensor including linkage means operatively connected at one end to said follower and at the other end to said movable support frame at a point laterally offset from said effective center about which the exiting web guide roller tilts.

5. In a web edge sensing and guiding apparatus for laterally guiding a longitudinally travelling web along a predetermined path of web travel including an exiting web guide roller extending transverse to a travelling web, a stationary support structure, means including a movable frame mounting said exiting web guide roller for movement relative to said stationary support structure in a plane of motion in which the exiting web guide roller is disposed generally tangent to a curved path about an effective arc center, said exiting web guide roller in the mid-position thereof extending perpendicular to the desired path of web travel and being arranged to have the travelling web extend partially therearound and exit therefrom in an exiting plane transverse to said plane of motion, a web edge sensor at a sensing location after the web exits from the exiting web guide roller for sensing lateral movement of the web edge in said exiting. plane from a preselected lateral position, power means operated by the web edge sensor for moving said movable frame and said exiting web guide roller in a direction to correct deviation of the web edge from said preselected lateral position, the improvement comprising: a rectilinear guideway on said stationary support structure paralleling said predetermined path of web travel and extending generally perpendicular to the exiting plane of the web, a follower slidable along said rectilinear guideway, means mounting said web edge sensor on said follower for movement therewith along a sensor shift path, which sensor shift path generally parallels a plane through said effective arc center and perpendicular to said exiting web guide roller when the latter is in its mid-position, and a linkage operatively connected to said follower and to said movable frame for moving said sensor along said sensor shift path in a direction and distance to substantially follow movement of the edge of the exiting web in a direction normal to said exiting plane while maintaining the sensor at a uniform spacing laterally of said predetermined path of web travel to thereby accommodate pass-line fluctuation of the exiting web at the sensing location.

, 6. A web edge sensing and guiding apparatus according to claim wherein said effective center is spaced from the side of said exiting web guide roller at which the web enters said exiting web guide roller and said linkage is operatively connected at one end to said follower and pivotally connected at the other end to said movable guide frame at a point laterally offset from said effective center.

7. A web edge sensing and guiding apparatus according to claim 6 including a second web guide roller mounted on said movable frame adjacent said effective center.

8. A web edge sensing and guiding apparatus according to claim 5 wherein said means mounting said web edge sensor includes an arm having one end pivotally attached to said follower for straight line motion of said one end of the arm with the follower, said arm extending from said follower crosswise of the path of web travel, means mounting the other end of said arm on said stationary support structure for combined pivotal and sliding movement in a direction crosswise of said path of web travel as said one end of the arm moves in said straight line motion, and means mounting the sensor on said arm for adjustment along the arm to adapt the web guide apparatus for webs of different width.

9. A web edge sensing and guiding apparatus according to claim 8 wherein the pivot axis of said other end of the arm is located approximately medially between opposite side edges of the web. 

1. In a web handling system for laterally guiding a web along a predetermined longitudinal path of travel in which the web engages a web guide apparatus mounted on a stationary support structure for tilting movement relative thereto in a plane of motion from a mid-position and exits from the web guide apparatus in an exiting plane transverse to the plane of motion of the web guide apparatus and wherein a web edge sensor is positioned at a sensing location to sense the lateral position of an edge of the web in said exiting plane and to operate a power means for tilting the web guide apparatus in said plane of motion in a direction to correct deviation of the web edge from a preselected lateral position, the improvement comprising means defining a rectilinear guideway on said stationary support structure paralleling said predetermined longitudinal path of web travel and extending transverse to the exiting plane of the web, means including a follower slidable along said guideway for supporting said web edge sensor, and means responsive to movement of said web guide apparatus in said plane of motion for moving said follower along said guideway in a direction and distance to cause said web edge sensor to substantially follow the component of movement of the exiting web at the sensing location that is substantially perpendicular to the exiting plane of the web while maintaining the sensor at a uniform spacing laterally of said predetermined longitudinal path of travel to thereby accommodate pass-line fluctuation of the exiting web caused by tilting of said web guide apparatus in said plane of motion.
 2. In a web handling system including a movable web guide apparatus mounted on a stationary support structure for tilting movement relative thereto in a plane of motion from a mid-position, and wherein the web exits from the web guide apparatus in an exiting plane transverse to said plane of motion, a web edge sensor at a sensing location after the web exits from said movable web guide apparatus to sense movement of the web edge laterally in said exiting plane from a preselected lateral position, and power means operated by the web edge sensor for tilting the movable web guide apparatus in a direction to correct lateral deviation of the web edge from said preselected lateral position, the improvement comprising means defining a rectilinear guideway on said stationary support structure paralleling said predetermined longitudinal path of web travel and generally perpendicular to the exiting plane of the web, means including a follower slidable along said guideway mounting the web edge sensor for substantially straight line movement along a sensor shift path which is generally perpendicular to said exiting plane when the web guide apparatus is in its mid-position, and means for moving said sensor along said sensor shift path as said movable web guide apparatus is tilted in said plane of motion to substantially follow movement of the edge of the exiting web in a direction perpendicular to said exiting plane to thereby accommodate pass-line fluctuation of the web at the sensing location.
 3. A web handling system according to claim 2 wherein the movable web guide apparatus has a movable support frame mounted on said stationary support structure for tilting movement in said plane of motion, and said means for moving the web edge sensor includes linkage means operatively connected to said follower and to said movable support frame.
 4. A web handling system according to claim 2 wherein the movable web guide apparatus includes an exiting web guide roller mounted by a movable support frame for tilting movement in said plane of motion about an effective arc center perpendicular to said plane of motion and spaced from said exiting web guide roller, said means for moving said web edge sensor including linkage means operatively connected at one end to said follower and at the other end to said movable support frame at a point laterally offset from said effective center about which the exiting web guide roller tilts.
 5. In a web edge sensing and guiding apparatus for laterally guiding a longitudinally travelling web along a predetermined path of web travel including an exiting web guide roller extending transverse to a travelling web, a stationary support structure, means including a movable frame mounting said exiting web guide roller for movement relative to said stationary support structure in a plane of motion in which the exiting web guide roller is disposed generally tangent to a curved path about an effective arc center, said exiting web guide roller in the mid-position thereof extending perpendicular to the desired path of web travel and being arranged to have the travelling web extend partially therearound and exit therefrom in an exiting plane transverse to said plane of motion, a web edge sensor at a sensing location after the web exits from the exiting web guide roller for sensing lateral movement of the web edge in said exiting plane from a preselected lateral position, power means operated by the web edge sensor for moving said movable frame and said exiting web guide roller in a direction to correct deviation of the web edge from said preselected lateral position, the improvement comprising: a rectilinear guideway on said stationary support structure paralleling said predetermined path of web travel and extending generally perpendicular to the exiting plane of the web, a follower slidable along said rectilinear guideway, means mounting said web edge sensor on said follower for movement therewith along a sensor shift path, which sensor shift path generally parallels a plane through said effective arc center and perpendicular to said exiting web guide roller when the latter is in its mid-position, and a linkage operatively connected to said follower and to said movable frame for moving said sensor along said sensor shift path in a direction and distance to substantially follow movement of the edge of the exiting web in a direction normal to said exiting plane while maintaining the sensor at a uniform spacing laterally of said predetermined path of web travel to thereby accommodate pass-line fluctuation of the exiting web at the sensing location.
 6. A web edge sensing and guiding apparatus according to claim 5 wherein said effective center is spaced from the side of said exiting web guide roller at which the web enters said exiting web guide roller and said linkage is operatively connected at one end to said follower and pivotally connected at the other end to said movable guide frame at a point laterally offset from said effective center.
 7. A web edge sensing and guiding apparatus according to claim 6 including a second web guide roller mounted on said movable frame adjacent said effective center.
 8. A web edge sensing and guiding apparatus according to claim 5 wherein said means mounting said web edge sensor includes an arm having one end pivotally attached to said follower for straight line motion of said one end of the arm with the follower, said arm extending from said follower crosswise of the path of web travel, means mounting the other end of said arm on said stationary support structure for combined pivotal and sliding movement in a direction crosswise of said path of web travel as said one end of the arm moves in said straight line motion, and means mounting the sensor on said arm for adjustment along the arm to adapt the web guide apparatus for webs of different width.
 9. A web edge sensing and guiding apparatus according to claim 8 wherein the pivot axis of said other end of the arm is located approximately medially between opposite side edges of the web. 